CI: Add poetry caching from libvcs, add python 2.7

[tony]

• Caching for poetry
• Python 2.7
• Skip running internal PR matrixes twice (hopefully)

[codecov]

Codecov Report

Merging #296 into master will decrease coverage by 0.09%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #296      +/-   ##
==========================================
- Coverage   86.99%   86.89%   -0.10%     
==========================================
  Files          16       16              
  Lines        1522     1526       +4     
==========================================
+ Hits         1324     1326       +2     
- Misses        198      200       +2     

Impacted Files	Coverage Δ
libtmux/window.py	83.33% <0.00%> (-0.50%)	⬇️
tests/test_tmuxobject.py	100.00% <0.00%> (ø)
libtmux/test.py	44.44% <0.00%> (+0.15%)	⬆️

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